It is difficult to accurately measure the emissivity or reflectivity of a surface especially at low grazing angles, but the need for reliable measurements is ever increasing, especially for military vehicles where these physical characteristics (and their control) are significant features for survival with respect to detection with modem optical (infrared {IR} and visible) seekers. Today, measurements of an entire vehicle are difficult, expensive, and require a carefully controlled or measured environment.
The present invention addresses a need to measure that IR directional emissivity and reflectivity of surfaces and coatings. The emissivity and bidirectional reflectivity play a major role in determining an airframe's total IR signature. Coatings are often designed to produce particular emissivity/reflectivity characteristics. While techniques exist to measure these properties for small samples in the laboratory, none of these methods are suitable for measuring the properties of the coatings once they are applied to a large airframe.
In U.S. patent application Ser. No. 08/184,556, we described a laboratory emissometer for measuring the emissivity of sample coatings. This device is useful only for small samples and is not suitable for measuring the signature of a surface or vehicle. It is, nevertheless, a useful tool for designing coatings. The emissometer operates by measuring the radiation eminating from the surface because of its temperature. This device is capable of measuring the emittance from normal to the surface (0.degree.) to a grazing angle of about 800.degree. off the normal spectrally and as a function of temperature between -65.degree. F. and 400.degree. F.
Surface Optics markets a protable measurement device that operates in the IR.
Modern aircraft have specifications and requirements for emissivity and reflectivity (based on the IR signature goals) that present new issues relative to the inspection techniques and tools required for manufacturing and maintenance. In particular, the IR signature is affected by the exterior coating reflectance and emittance. The performance of the coating can be sensitive to variations in the coating materials, application processes, and to environmental exposure. To assure that an aircraft meets IR specifications, IR coating performance must be tested as applied on the air vehicle surface. Such measurements must be repeated on a regular maintenance schedule and after repairs to assure continuing signature performance of the aircraft. These measurements must be nondestructive, rapid, and require minimal skill level and training. Measurement devices should be easily portable (hand-held if possible), affordable, rugged, and require little support (standard power, room temperature detectors).
NDE (nondestructive evaluation) measurements must provide sufficient information to assess whether the surface meets coating and vehicle specifications. It is important that the information be of sufficient content and quality, without incurring costs associated with collecting unused information. In addition, the instrument must be able to collect data and indicate pass/fail to the operator.